SRAM (Static Random Access Memory) is one of high-speed volatile memories, and is often used in large-scale integrated circuits for today's computers. Each one SRAM is basically formed with six transistors, and can perform higher-speed operations with smaller areas in association with miniaturization of transistors. However, miniaturization of transistors is becoming more and more difficult today. As a result, noise margins are becoming smaller due to device-to-device variations of transistors, and operations are becoming unstable.
To counter this problem, there have been SRAMs in which gate oxide films are replaced with films made of another material, the polarity of the material varies with the magnitude of the gate voltage so that the threshold voltage of transistors varies. In such a SRAM, ferroelectric field effect transistors (hereinafter also referred to as FeFETs) are used as the six transistors, to successfully increase the threshold voltage of each transistor and widen the noise margin of the SRAM by 60% or more. Characteristically, the substrates of the NMOS transistors and the PMOS transistors are connected to a source voltage VDD and a ground voltage VSS, respectively, though it is the other way around in conventional cases. Through the connections, the polarization in the ferroelectric material can have the opposite charge configuration of that of the gate voltage. This is the principal reason for the higher threshold voltages.
In a SRAM including the above described FeFETs, a material that is not used in conventional CMOS manufacturing processes is used as the ferroelectric material. Examples of ferroelectric materials that can be used for the FeFETs include lead zirconate titanate (hereinafter also referred to as PZT), strontium bismuth tantalate (hereinafter also referred to as SBT), and bismuth lanthanum titanate (hereinafter also referred to as BLT). Expressed as Pb(Zr, Ti)O3, PZT contains harmful lead (Pb), and therefore, cannot comply with environmental regulations. Also, SBT is expressed as SrBi2Ta2O9, and needs to be crystallized at a high temperature of 700° C. or higher to achieve ferroelectricity. It is difficult to grow a thin film in an a-axis direction having spontaneous polarization. Furthermore, the residual polarization is 25 μC/cm2, which is relatively small. BLT is expressed as (Bi, Ln)4Ti3O12, and it is difficult to crystallize BLT by controlling orientations. It should be noted that Ln represents La, Nd, Pr, or the like. Therefore, it is difficult to use ferroelectric materials in today's industries, due to small residual polarization and high coercive fields or the like.